Quantitative knowledge of T1 relaxation time provides information that is useful for identifying and analysing a variety of pathological conditions. Quantitative knowledge of T1 relaxation also facilitates quantitative perfusion measurements. Conventionally, acquiring quantitative knowledge of T1 relaxation times has been challenging in large volumes that are affected by motion (e.g., abdomen). Conventional attempts to acquire full volume abdominal T1 maps in multiple breath holds have used a variable flip angle approach. Unfortunately, conventional variable flip angle approaches have been sensitive to B1 field inhomogeneity and have been compromised by motion between breath-holds. Quantifying other magnetic resonance (MR) parameters (e.g., T2 relaxation, diffusion) has also been difficult, if even possible, to perform for large volumes that may experience motion.
Conventional attempts to acquire quantitative knowledge of MR parameters like T1 relaxation times may have included applying a Look-Locker pulse sequence, which is also known as a T1-scout method, to perform rapid T1 mapping. FIG. 1 illustrates a two dimensional Look-Locker pulse sequence. In general, a Look-Locker pulse sequence includes a preparation phase followed by multiple image acquisition phases. Unfortunately, due to the large volume to be covered, a Look-Locker pulse sequence applied with a Cartesian readout has not provided 3D T1 maps with adequate resolution in a clinically relevant scan time.
The Look-Locker method dates back to a spectroscopic one-shot method proposed by Look and Locker in 1968 (D. C. Look and D. R. Locker, Phys. Rev. Lett. 20, 987 (1968). Enhancements to the initial spectroscopic one shot method were described in Time Saving In Measurement Of NMR and EPR Relaxation Times, Look et al., The Review of Scientific Instruments, Volume 41, Number 2, February 1970. The paper described that by producing a train of absorption or dispersion signals (e.g., continuous-wave magnetic resonance) or free induction decays (e.g., pulsed magnetic resonance) it was possible to save time in spin-lattice relaxation measurements because it was not necessary to wait for equilibrium magnetization before initiating the train. The relaxation time was calculated from the train using a converging iteration.
The Look-Locker method was adapted to quickly sample the recovery after a preparation pulse during the recovery period or transient phase, as described in A Single-Scan Fourier Transform Method For Measuring Spin-Lattice Relaxation Times, Kaptein et al., Journal of Magnetic Resonance, Volume 24, Issue 2, November 1976, pages 295-300. The Kaptein method was then adapted by Graumann et al. to produce the TOMROP imaging sequence (T One by Multiple Readout Pulses). In the TOMROP approach, the multiple samples of a particular recovery after radio frequency (RF) preparation correspond to separate images. To acquire a complete data set for each image, the whole sequence is repeated numerous times, where a repetition fills the next line of k-space for an image. This may lead to lengthy acquisition times. This may also lead to the situation where different images have different unique delay times.
The Look-Locker (LL) method has been optimized and refined over the years for different purposes. For example, improved RF preparation pulses have been described by Been et al., in Serial Changes In The T1 Magnetic Relaxation Parameter After Myocardial Infarction In Man, BR Heart J 1988 29: 1-8. This paper described using a low field resistive nuclear magnetic resonance imaging system to study the in vivo changes in the relaxation parameter T1 in the heart. T1 maps were constructed from transverse and coronal images. Calculated T1 maps were obtained by an interleaved saturation-recovery and inversion-recovery pulse sequence with a time from inversion of 200 ms. Inversion was obtained with an adiabatic fast passage inverting pulse rather than the conventional 180 degree pulse. This adiabatic fast passage inverting pulse efficiently inverted all the nuclei in a volume for which a T1 map was to be produced.
The Look-Locker method has been adapted to include echo-planar imaging (EPI) in the inversion recovery as described by Ordidge et al., in High-speed multislice T1 mapping using inversion-recovery echo-planar imaging, Magn Reson Med 1990:16(2):238-245. Ordidge interleaved EPI readouts for eight different slices after an inversion pulse. The sequence was then repeated and the slice order was changed to achieve a range of inversion times for slices. Look-Locker with EPI was later applied in vivo in less than three seconds using a modified blipped EPI technique. While this technique was faster, precision and accuracy were compromised. Look-Locker with EPI facilitated acquiring an entire image at each point on a single recovery of longitudinal magnetisation after a saturation pulse. The Look-Locker with EPI technique was optimised as described by Freeman et al., in Optimization Of The Ultrafast Look-Locker Echo-Planar Imaging of T1 Mapping Sequence, Magnetic Resonance Imaging, Vol. 16, No. 7, pp 765-772, 1998. This paper described how the measurement of T1 was important for studying in-flow perfusion and for studying dynamic contrast agent techniques.
An LL-EPI sequence includes a preparation phase (e.g., inversion pulse) followed by multiple image acquisition units. An image acquisition unit includes a low flip-angle RF pulse (e.g., a magnetization sample pulse) and the EPI module. T1 maps are calculated by fitting the signal to an image, pixel by pixel, to the recovery equation integrated over the sample pulse slice profile. Adaptations to the LL method included the modified Look-Locker (MOLLI) method. Comparisons of the LL method and the MOLLI method have been described in, for example, Myocardial T1 Mapping With MRI: Comparison of Look-Locker and MOLLI Sequences, Nacif et al., Journal of Magnetic Resonance imaging, 34; 1367-1373 (2011).
Magnetic resonance imaging (MRI) provides highly detailed anatomical information. Dynamic contrast-enhanced (DCE) MRI monitors the transit of contrast materials (e.g., gadolinium (Gd) chelates) through various regions (e.g., kidneys, liver). MRI using DCE may experience several stages. For example, at a first time, a bolus of contrast agent may arrive at a first location and produce a series of enhancement effects. By analyzing the enhancement at various time points after administration of contrast agent, clinically relevant parameters including blood flow, perfusion, and blood volume may be measured. However, acquiring sufficient signals to perform quantitation that is sufficient to support meaningful functional analysis requires a combination of spatial resolution and temporal resolution that has not been conventionally available.
Conventionally, different methods have been used to quantify information acquired by MRI. These methods include the upslope method, semi-quantitative parametric methods, and de-convolution methods. Unfortunately, the temporal resolution provided by conventional MRI systems may not have been sufficient to support high resolution three-dimensional (3D) T1 mapping for a large volume (e.g., abdomen). Additionally, applying conventional under-sampling to improve temporal resolution may have negatively impacted spatial resolution to the point where image quality fell below a desired level.
A pulse sequence is a preselected set of defined RF and gradient pulses that may be repeated many times during a scan. The time interval between pulses and the amplitude and shape of the gradient waveforms control NMR signal reception. Pulse sequences are characterized by parameters including repetition time (TR), echo time (TE), inversion time (TI), flip angle (FA), and other parameters. Look-Locker as implemented in T1 -scout is a gradient recalled echo (GRE) sequence. A gradient echo is generated using a pair of bipolar gradient pulses. There may be no refocusing 180 degree pulse and the data may be sampled during a gradient echo. The gradient echo is achieved by dephasing spins with a negatively pulsed gradient before the spins are rephased by an opposite gradient with opposite polarity, which generates the echo. An excitation pulse may be referred to as an alpha pulse α. The α pulse tilts the magnetization by a flip angle typically between 0 degrees and 90 degrees. The flip angle may be varied during data acquisition. In an ultrafast GRE sequence, TR and TE may be so short that tissues have a limited imaging signal and limited contrast. For example, TR may be less than 5 ms and TE may be less than 1 ms. Thus, in an ultrafast GRE sequence, magnetization may be prepared during the preparation module using, for example, a 180 degree inversion pulse. FIG. 2 shows an example ultrafast GRE pulse sequence. Look-Locker may also be implemented as, for example, an inversion recovery (IR) sequence. Unlike conventional inversion recovery (IR) sequences, multiple lines in k-space may be acquired after a single inversion pulse.
Conventional studies of large volumes have typically employed T1-weighted, GRE sequences. T1 refers to spin-lattice relaxation, T2 refers to spin-spin relaxation. 3D acquisitions may have provided continuous coverage of a volume but only at the expense of longer acquisition times. When longer acquisition times are required, issues associated with movement in the volume are exacerbated. 3D T1 mapping within one breath-hold has typically been challenging. Thus, two-dimensional (2D) images have typically been acquired with higher temporal and spatial resolution. However, the 2D image approach may have been limited to a single representative slice or selected slices, which precluded whole volume analysis. Achieving higher temporal and spatial resolution facilitates achieving greater precision, accuracy, and image quality.
Kinetic modeling involves converting an MRI signal into a gadolinium (Gd) concentration. This conversion has been challenging because magnetic resonance (MR) signal intensity varies with contrast agent concentration, pulse sequence parameters, pre-contrast relaxation times, blood flow velocity, and other factors. Additionally, the relationship between signal and concentration is non-linear. Conventional spatial and temporal resolution may have been insufficient to provide adequate signal for meaningful functional analysis involving kinetic modeling.